FABRICATION AND CHARACTERISATION OF SUPERSTRATE COPPER ZINC TIN SULPHIDE (Cu2ZnSnS4) THIN FILMS SOLAR CELLS

ABSTRACT

Greenhouse gases emitted from burnt fossil fuels are major contributors to global warming which currently threatens existence of life. Hence, the global campaign for the replacement of fossil fuels with solar energy. The Cu2ZnSnS4 (CZTS) thin film has emerged as a promising material for photovoltaic absorption layer in the fabrication of solar cells. Various methods of deposition have been used to fabricate superstrate CZTS thin film solar cells. However, there is limited information on the use of sol-gel method which is known to perfectly control chemical composition for the fabrication of this type of solar cells. In this study, a superstrate structured CZTS thin film solar cell was fabricated using the sol-gel method and thereafter characterised. The possibility of using paper tape during the fabrication process instead of photomask which is not always readily available was also investigated. Superstrate structured CZTS thin film solar cells were fabricated using atmospheric pressure chemical vapour to deposit SnO2:F thin film (as window layer). Also, spin coating method was used to deposit CdS thin film (as buffer layer) from the sol-gel of cadmium acetate, 2-methoxy ethanol, thiourea and polyethylene glycol. Sol-gel sulphurising method was used to deposit Cu2ZnSnS4 thin film (as absorber layer) from CuCl2, ZnCl, SnCl2 and thiourea (as sulphur source). Layers of the film were masked using paper tape, optimised and characterised following standard procedures before being used in the fabrication of (SLG)/FTO (SnO2:F)/Ag/CdS/CZTS/Al solar cell. The deposited films were characterised using scanning electron microscope for surface morphology, Jandel universal four point probe for sheet resistance, MD-10 X-ray Diffractometer for crystallography and AvaSpec-2048 UV-VIS spectrophotometer for optical properties. The CZTS thin film solar cells of active area 2.25cm2 were examined for their efficiencies using the Newport solar simulator under irradiation of 100 mW/cm2 . The mean of the efficiencies of CZTS solar cells from sol-gel method were compared with previous work on screen printing and doctor-blade methods. The SnO2:F thin film obtained had a transmittance of 80% in the UV-Visible region of the electromagnetic spectrum and a sheet resistance of 15 Ω/sq which is within acceptable values for a window layer. The calculated energy band gap of the CdS and CZTS films from the transmittance and reflectance data were 2.40 eV and 1.51 eV respectively. This value for CZTS is closest to near-optimal direct band gap value (1.50eV) when compared with those obtained via screen printing (1.40eV) and doctor blade (1.41eV) methods. The X-ray diffraction pattern of the CdS film exhibit hexagonal structure with (002) orientation while CZTS film had kesterite structure with (112) orientation. The absorption co-efficient of CZTS film in the visible region was 104 cm-1 . The X-ray diffraction pattern showed sharp peaks whereas broadened peaks were observed in the X-ray diffraction pattern obtained via doctor blade method. The fabricated solar cells with buffer layer thicknesses of 40 nm and 60 nm had efficiencies of 0.28 0.01% and 0.13 0.01% respectively when compared with screen printing (0.53%) and doctor-blade (0.55%). Superstrate Cu2ZnSnS4 solar cells have been successfully fabricated using sol-gel spin coating method and paper tape in lieu of photomask.